This article was originally published conversation. Post contributed this article to Space.com Expert Voices: Editorial and Insights.
John ConwayProfessor of Physics, University of California, Davis
“You can do it quickly, you can do it cheaply, or you can do it right. We did it right.” These were some of the early observations of David Toback, the head of the collider detector at Fermilab, when he announced the results of a ten-year experiment to measure the mass of a particle called the W boson.
I’m a high-energy particle physicist, I’m part of a team of hundreds of scientists who built and operated the Collider detector at Fermilab in Illinois – known as CDF.
After trillions of collisions and years of data collection and number-crunching, the CDF team discovered that the W boson has a slightly greater mass than expected. Although the discrepancy is slight, the findings are presented in a research paper published in Science on April 7 on electrified particle physicists. If the measurement is correct, this is another strong sign that there are missing pieces of the physical puzzle of how the universe works.
A particle that carries the weak force
The Standard Model of particle physics is the current best scientific framework for the fundamental laws of the universe and describes three fundamental forces: the electromagnetic force, the weak force, and the strong force.
Strong force holds atomic nuclei together. But some nuclei are unstable and undergo radioactive decay, slowly releasing energy by emission of particles. This process is driven by the weak force, and since the early 1900s, physicists have attempted to explain why and how atoms decay.
According to the Standard Model, forces are transmitted by particles. In the 1960s, a series of theoretical and experimental developments suggested that the weak force was transmitted by particles called W and Z bosons. He also postulated that the third particle, the Higgs boson, was what gives all other particles—including the W and Z bosons—mass.
Since the emergence of the Standard Model in the 1960s, scientists have been studying the list of predicted, hitherto unknown particles, and measuring their properties. In 1983, two experiments at CERN in Geneva, Switzerland captured the first evidence for the existence of the W boson and appeared to contain roughly the mass of an intermediate-sized atom as bromine.
In 2000, there was one piece missing to complete the Standard Model and put everything together: the Higgs boson. I helped search for the Higgs boson in three successive experiments, and finally discovered it in 2012 at CERN’s Large Hadron Collider.
The standard model was complete and all of our measurements fit perfectly with expectations.
Measurement of the boson W
The standard shape test is fun – you just crush particles with very high energy. These collisions produce heavier particles for a while and then decay back into lighter particles. Physicists are using massive, high-sensitivity detectors at places like Fermilab and CERN to measure the properties and interactions of particles produced by these collisions.
In CDF, 1 in 10 million W bosons are produced when a proton and antiproton collide. Antiprotons are the antimatter version of protons, having exactly the same mass but opposite charge. Protons are made of smaller fundamental particles called quarks, and antiprotons are made of antiquarks. It’s a collision between quarks and antiquarks to form W bosons. So physicists track their decay energy to measure the mass of the W bosons.
In the 40 years since scientists discovered evidence of the W boson, successive experiments have achieved more accurate measurements of its mass. But only since the Higgs boson has been measured — since it gives mass to all other particles — researchers have been able to check the measured mass of the W bosons against the mass predicted by the Standard Model. Expectations and experiences have always been identical – until now.
Fermilab’s CDF detector is excellent at accurately measuring W bosons. From 2001 to 2011, the accelerator collided protons with antiprotons trillions of times, producing millions of W bosons and recording as much data as possible from each collision.
Fermilab. The team published the first results using a fraction of the 2012 data. We found the lock to be a bit off, but it’s close to predicting. The team spent a decade painstakingly analyzing the entire data set. The process involved many internal checks and required years of computer simulation. To avoid any bias in the analysis, no one was able to see any results until the entire calculation was completed.
When the physicist finally saw the result on April 7, 2022, we were all shocked. Physicists measure the masses of elementary particles in units of millions of electron volts – abbreviated to MeV. The mass of the W boson turns out to be 80,433 MeV – 70 MeV more than the standard model predicts. This may seem like a small excess, but the measurement is accurate to within 9 meV. This is a deviation of approximately eight times the margin of error. When my colleagues and I saw the result, our reaction was “Wow!”
What does this mean for the standard model
The fact that the measured mass of the W boson does not match the mass expected in the Standard Model can mean three things. Either the math is wrong, the analogy is wrong, or something is missing from the Standard Model.
First, the math. To calculate the mass of the W boson, physicists use the mass of the Higgs boson. CERN experiments have allowed physicists to measure the mass of the Higgs boson in the order of a quarter of a percent. In addition, theoretical physicists have been working on calculations of the mass of W bosons for decades. While the math is complex, the prediction is powerful and unlikely to change.
The next possibility is a defect in the experiment or analysis. Physicists around the world are already revising the result to try and drill holes in it. Moreover, future experiments at CERN may eventually reach a more accurate result that will confirm or refute Fermilab blockade. But in my opinion, experience is as good a metric as it is currently.
This leaves the last option: there are unexplained particles or forces causing the upward shift in the mass of the W boson. Even before this measurement, some theorists had already proposed new particles or possible forces that would lead to the observed shift. In the coming months and years, I look forward to a series of new articles that seek to explain the intriguing mass of W bosons.
As a particle physicist, I’m confident to say there must be more physics waiting to be discovered beyond the Standard Model. If this new result is correct, it will be the latest in a string of results showing that the Standard Model and real-world measurements generally do not match up exactly. It is these mysteries that give physicists new clues and new reasons to continue the search for a more complete understanding of matter, energy, space and time.
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